Start Half-Life using "hl.exe -dev -console -game gearbox.
You may do this by modifying the windows shortcut properties for Half-Life OpFor and adding "-dev -console -game gearbox" to the end of the "Target" (or file to run).

Hidden Audio
Approach the Boss of Opposing Force to hear backwards audio composed of the phrase "To win the game you must kill me Randy Pitchford" when played correctly. Randy Pitchford is the leader of the development team at Gearbox Software.